1. Field of the Invention
The present invention relates to a guard trace, and more particularly, to a guard trace pattern reducing far-end crosstalk.
2. Description of the Related Art
Far-end crosstalk is noise generated by electromagnetic interference between adjacent signal lines. When high-frequency signals are transmitted through two signal lines running in parallel with each other, the signals transmitted through one of the two signal lines or the two signal lines mutually interfere with each other. Due to the far-end crosstalk generated by the mutual interference, signal transmission losses increase.
Between the two signal lines, capacitive interference due to mutual capacitance and inductive interference due to mutual inductance occur. When a difference between the capacitive interference and the inductive interference occurs, the far-end crosstalk in a receiving terminal is generated.
The far-end crosstalk VFEXT(t) in the receiving terminal can be represented by Equation 1.
                                          V            FEXT                    ⁡                      (            t            )                          =                              l            2                    ⁢                      (                                                            C                  m                                                  C                  t                                            -                                                L                  m                                                  L                  s                                                      )                    ⁢                                    ∂                                                V                  a                                ⁡                                  (                                      t                    -                    TD                                    )                                                                    ∂              t                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Here, Cm denotes a mutual capacitance between a transmitting terminal and the receiving terminal, and Lm denotes a mutual inductance between the transmitting terminal and the receiving terminal.
Referring to Equation 1, it can be seen that the far-end crosstalk V FEXT(t) in the receiving terminal is proportional to a value obtained by multiplying half a length of a transmission line 1, a time derivative of a transmission signal Va consideration of a time delay TD of the transmission line (∂Va(t-TD)/∂t, and a difference between the capacitive interference and the inductive interference.
Here, the capacitive interference is a value obtained by normalizing the mutual capacitance Cm by C1 (a self capacitance plus the mutual capacitance), and the inductive interference is a value obtained by normalizing the mutual inductance Lm by Ls (a self inductance).
In transmission lines in uniform media such as strip lines, an amount of capacitive interference and an amount of inductive interference between the strip lines are the same, so that the far-end crosstalk in the receiving terminal ideally has a value of 0. However, inductive interference between micro strip lines formed on a printed circuit board is greater than capacitive interference therebetween, so that the far-end crosstalk in the receiving terminal has a negative value.
So as not to generate the far-end crosstalk that does not have a value of 0, a guard trace is disposed between signal lines. The guard trace is a dummy signal line provided between two adjacent signal lines. The guard trace is not associated with signal transmission and performs a function of reducing mutual interference between high-speed signals transmitted through the two signal lines.
FIG. 1 illustrates a structure of a conventional guard trace.
Referring to FIG. 1, the conventional guard trace is disposed between two signal lines Aggressor-line and Victim-line. In FIG. 1, Aggressor line is a transmission line which a signal is applied to an end thereof, Victim-line is a transmission line which a signal is not applied to an end thereof. In addition, Vsl is a source voltage, Val is a Aggressor voltage, Vfe is a far-end crosstalk voltage. And TD is a Propagation Time Delay. Resistances (hereinafter, referred to as terminal resistances R0) provided to both ends of the two signal lines have the same resistance value as a resistance component of characteristic impedances (Z) of the signal lines. A resistance value R0 of the terminal resistance of the guard trace is the same as that of the two signal lines.
FIG. 2 is a cross-sectional view illustrating a printed circuit board including the guard trace illustrated in FIG. 1. The printed circuit board, for example, a “FR4” (Flame Retardent 4) board of which dielectric constant (∈) is 4.5. The printed circuit board may be a board of which dielectric thickness is 8 mil bottom copper thickness is 1.4 mil, and upper copper thickness is 0.7 mil. Referring to FIG. 2, the conventional Aggressor-line, Victim-Line and guard trace have a width of 14 mil and a thickness of 0.7 mil. The guard trace, Aggressor-line, and Victim-line are separated each other at an interval of 14 mil. Here, 1 mil denotes 1/1000 inch.
The conventional guard trace has an advantage of reducing a value of the mutual inductance. However, only a part of the amount of the far-end crosstalk in the receiving terminal is reduced, so that a system performance cannot be significantly increased.